Metathesis polycondensation chemistry has been employed to control the crystalline morphology of a series of 11 precision-branched polyethylene structures, the branch being placed on each 21st carbon and ranging in size from a methyl group to an adamantyl group. The crystalline unit cell is shifted from orthorhombic to triclinic, depending upon the nature of the precision branch. Further, the branch can be positioned either in the crystalline phase or in the amorphous phase of polyethylene, a morphology change dictated by the size of the precision branch. This level of morphology control is accomplished using step polymerization chemistry to produce polyethylene rather than conventional chain polymerization techniques. Doing so requires the synthesis of a series of unique symmetrical diene monomers incorporating the branch in question, followed by ADMET polymerization and hydrogenation to yield the precision-branched polyethylene under study. Exhaustive structure characterization of all reaction intermediates as well as the precision polymers themselves is presented. A clear change in morphology was observed for such polymers, where small branches (methyl and ethyl) are included in the unit cell, while branches equal to or greater in mass than propyl are excluded from the crystal. When the branch is excluded from the unit cell, all such polyethylene polymers possess essentially the same melting temperature, regardless of the size of the branch, even for the adamantyl branch.

Branching out: The mobility of linear polymers changes upon branching, which has a pronounced effect on processability and drawability. Regularly branched model polyolefins were studied by advanced solid-state NMR spectroscopy, and twist defects around the branches in the crystalline regions are identified. For lower branch content, the twisting motions are decoupled; for higher content, collective motion is found (see picture).

A structural investigation of linear ethylene-covinyl amine (EVAm) copolymers having a primary amine branch on every 9th, 15th, 19th, or 21st carbon along the ethylene backbone has been completed using step polymerization chemistry. Acyclic diene metathesis (ADMET) polymerization has been used with symmetrical α,ω dienes containing protected amine groups to afford polymers with exact primary structures and constant methylene run lengths between branches. The effects of subtle structural changes such as the ethylene run lengths between amine branches can be observed and used to correlate structure property relationships. NMR and FT-IR techniques are used to characterize and verify the excellent structural control this synthetic approach provides over traditional chain polymerization techniques. Thermal decomposition of these copolymers is shown to additionally support polymer structure while differential scanning calorimetry demonstrates crystallinity in the polymers with an amine on every 15th and 21st carbon, whereas the polymer with an amine on every ninth carbon is amorphous. Variations of the physical and spectral properties are discussed as a consequence of the amine branch spacing, protection, and saturation of the ethylene backbone.

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The needled/stitched dual-scale interlocking structure is a new fabric structure. However, the effect of stitching on the interlaminar bonding behavior of needled composite has not been elucidated. In this work, the needled/tufted dual-scale interlocking composite was prepared, and experimental and numerical simulation of Mode I interlaminar property was carried out. The influence of stitching on the interfacial bonding behavior of needled composite was revealed. Results showed that the implantation of tufted fibers greatly improved the Mode I interlaminar property of the needled composite. The maximum experimental load of double cantilever beam (DCB) was 129.32 N, which increased by 248.53% compared to needled composites. And GIC increased by 3 times. Moreover，within a certain range, with the increase of the length of the head of the tufted fiber bundle, the interlaminar performance of Mode I was effectively improved. The typical failure mode of needled/tufted dual-scale interlocking composite were matrix cracking, brittle fracture of needled fiber bundles, pulled out and brittle fracture of tufted fiber bundles. Moreover, the Tri-cohesive zone models (TCZMs) was proposed, and DCB simulation of needled/tufted dual-scale interlocking composite was conducted. The simulation of load-displacement curves was well closed to the experiment.The error range was within 7%.